(PSN)FoGsesipod

MR progression is tied to ranks of weapons/frames/etc if you had a weapon on PC that was level 30 that you didnt on ps4, then that progress will be added to the total MR progression you have (alongside all 21 levels from progression done on playstation) Aka, it doesnt matter which one is your primary account for MR.

You duplicated your items from PC to Console with your migration back in the day. merging now, would effectively combine all of these items into a single account... meaning you would "cheat" the system technically and get 2 of all the items you have. Example: say you had 1000 polymer bundles on PC when you sent it to PS. now you have 1000 on PC and PS, if you combine these accounts, you now have 2000 on a single account. 2000, that you never grinded for. as you only ever grinded for 1000. Now apply this to frames, weapons, companions, frame slots, weapon slots, companion slots, mods, rivens,....

It unfortunately isn't this simple, purchasing tennogen on console gives you the right to the product you purchased with platinum on consoles (see how they mention any platinum purchased tennogen is universal across all consoles) however buying tennogen with real money like on pc is a different license to the product you are buying. This is all just legal issues, you cannot without either DE/sony/microsoft/nintendo/steam/tennogen artists getting screwed, have tennogen items purchased with money be allowed for use on platforms where tennogen is purchased with platinum

"As long as the main account that you are linking had the stuff before the 24th" paraphrased

Destiny 2 is also a cross save game, so ill go ahead and make a comparison from it for you: Just because you bought forsaken on playstation - giving a cut of the price to sony - doesn't mean that steam has to honor your purchase. Yes, in destiny 2 you do not keep the dlcs you paid for on other platforms when using cross save, you have to buy the dlc on every platform. It is literally the same situation here. And no, just because on of them is called "DLC" and the other "Tennogen" doesn't make them different, you still pay money for these items, and a platform takes a cut of the price you paid for it. Also, no, just because you are a customer for warframe doesn't mean you deserve any legal documentation or official essays as to why DE cannot allow you to screw over steam/sony/microsoft/nintendo/tennogen artists. If you would like to get sued and face legal battles, go somewhere else.

This screws over Sony/Microsoft/Nintendo because they aren't getting their cut of the item. Its a legal reason why. We are luckly DE has negotiated and talked down the three companies to allow for unified platinum wallets and previous tennogen purchases to be unified

Hope this answers your question